Power control system and method

ABSTRACT

A power system includes an energy harvesting device, a battery coupled to the energy harvesting device, and a circuit coupled to the energy harvesting device and the battery. The circuit is adapted to deliver power to a load by providing power generated by the energy harvesting device to the load without delivering excess power to the battery and to supplement the power generated by the energy harvesting device with power from the battery if the power generated by the energy harvesting device is insufficient to fully power the load. A method of operating the power system is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 11/061,706, filed on Feb.17, 2005 now U.S. Pat. No. 7,132,757.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with Government support under contract numberDE-FC36-04GO14001 awarded by Department Of Energy. The Government hascertain rights in the invention.

BACKGROUND

The invention relates generally to the field of energy harvesting, andmore particularly, to energy harvesting circuits for reducing batterydrain.

Energy harvesting is a process for recovering power that is otherwisedissipated or lost in a system. For example, energy harvesting may beused to obtain energy from solar activity, wind, thermal sources, waveaction, water currents, and the like. Similarly, energy may be harvestedfrom other sources, such as motor vibrations, pressure changes in thesoles of shoes, and the like. In many systems, harvested energy may beused in conjunction with battery power to provide power to a load, suchas a sensor or the like. Harvested energy may be used to power the loadunder normal conditions, with power from the battery being used as asupplement during periods when harvested energy is insufficient to fullypower the load. Such systems may extend the useful lifetime of thebattery.

Some systems utilize excess harvested energy to recharge the battery inan attempt to further maximize battery life. However, this requires arechargeable battery for functioning. One drawback with rechargeablebatteries is that they have low useful battery life compared tonon-rechargeable batteries if there is insufficient harvested energy torecharge the battery.

Other systems utilize a harvesting energy source along withnon-rechargeable batteries. In such systems, non-rechargeable batteriesare primarily used to prolong the continuous delivery of power to thesystem. These systems may accidentally charge the battery or deliver anaccidental charging current to the battery. Because, the life ofnon-rechargeable batteries may be affected if they are charged or ifthey receive a charging current, such systems are not effective forlong-life applications. An improved circuit for utilizing harvestedenergy to power a load in conjunction with a battery is desirable.

SUMMARY

In accordance with one aspect of the present technique, a power systemis provided. The power system includes an energy harvesting device, abattery coupled to the energy harvesting device, and a circuit coupledto the energy harvesting device and the battery. The circuit controlsdelivery of power to a load by (i) providing power generated by theenergy harvesting device to the load without delivering excess power tothe battery, and (ii) supplementing the power generated by the energyharvesting device with power from the battery if the power generated bythe energy harvesting device is insufficient to fully power the load. Amethod of operating the power system is also provided.

These and other advantages and features will be more readily understoodfrom the following detailed description of preferred embodiments of theinvention that is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary power system in accordancewith aspects of the present technique.

FIG. 2 is a block diagram of a motor driven system in accordance withaspects of the present technique.

FIG. 3 is a schematic diagram of a power system in accordance withaspects of the present technique.

FIG. 4 is a schematic diagram of an alternative embodiment of a powersystem in accordance with aspects of the present technique.

FIG. 5 is a schematic diagram of another alternative embodiment of apower system in accordance with aspects of the present technique.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In subsequent paragraphs, various circuits, systems, and methods forimplementation of different aspects of the power system will bedescribed in greater detail. FIG. 1 is a block diagram of an exemplarypower system 10 in accordance with aspects of the present technique. Thepower system 10 comprises an energy harvesting device 12 that providespower to a load 14. The energy harvesting device 12 may be apiezoelectric transducer or a generation device that converts varioustypes of mechanical vibrations or disturbances into electrical power.For example, vibrations from pumps, turbines, engines, bridges whenvehicles travel across, and the like may be utilized depending onspecific applications. In alternative implementations, an acoustictransducer or a transducer that converts light energy into electricalenergy may be employed to generate electrical power. In anotherimplementation, a thermal transducer designed to detect various degreesof thermal gradients may be utilized. The detected thermal gradient maybe converted into electrical energy and may be utilized to power theload 14. Similarly, other transducers that can provide electrical energyfrom any other form of energy may also be utilized.

A rectifier 16 converts varying or alternating current (ac) provided bythe energy harvesting device 12 into a direct current (dc) signal. Thespecific configuration details of the rectifier 14 are matters of designchoice and should not be considered limitations to the scope of thepresent technique. By way of example and not limitation, half-wave,full-wave, or voltage doubling rectifiers may be used as well as voltagemultiplying circuits in general. Examples of voltage multiplyingcircuits include Cockroff and Walton voltage multiplying circuits. Therectified power output of the rectifier 16 is provided to the load 14. Abattery 18 supplements the power provided by the energy harvestingdevice 12, such that if the amount of power required by the load 14 isnot provided by the energy harvesting device 12, the battery 18 providesthe load 14 with the deficient power. One example of a battery that maybe utilized is a Lithium-ion non-rechargeable battery. Furthermore, thepower system 10 may be designed to avoid any accidental charging of thebattery 18.

Referring generally to FIG. 2, a diagrammatical view of the power system10 implemented in a motorized system 20 is shown. As illustrated, amotor 22 drives a motor-driven-system 24, and the power system 10 iscoupled to the motor 22 for monitoring. Vibrations, for example, bearingvibrations, generated by the motor 22 are converted into electricalpower by the energy harvesting device 12 in the power system 10. As hadbeen previously discussed with respect to FIG. 1, the load 14, which isa wireless sensor in this exemplary embodiment, is powered by theharvested power generated by the energy harvesting device 12. However,when the power generated by the vibrations is not enough to power thewireless sensor 14, the battery 18 provides the deficient power.

The wireless sensor 14 may provide a signal indicative of the status ofthe motor 22. For example, the amount of vibrations generated by themotor 22 increases with aging of the motor 22. This change in vibrationsmay be detected and transmitted to a remote location by the wirelesssensor 14. Alternatively, the amount of vibrations may be detected andtransmitted to the remote location for further processing, such asmaintenance of motor statistics, periodic maintenance checks, currentmotor use statistics, and the like. When the vibrations increase in themotor 22, the harvested power is higher and correspondingly lesseramount of power is drawn from the battery at a stage when the wirelesssensor 14 requires the maximum power.

The embodiment described hereinabove is just one of the manyimplementations in which embodiments of the present technique may beemployed. However, the system may be modified to incorporate any numberof variations, alterations, substitutions or equivalent arrangements.For example, the power system 10 may be incorporated in the tire of avehicle, wherein a low power wireless sensor may be designed to transmita plurality of data, including air pressure within the tire, temperatureof the tire, and the like. Similarly, the power system 10 may beincorporated in a pedometer, railroads, ductwork in buildings, householdappliances that may serve as vibration sources, for providing datarepresentative of one or more parameters of the respective equipment.

FIG. 3 is a schematic diagram of one implementation of the power system10 in accordance with aspects of the present technique. A vibrationsource 12 is coupled with a piezoelectric beam 26 that is mechanicallytuned, to the expected vibration frequency, with a tuning mass 28. Thechoice of material and other specifications for the tuning mass 28 maydepend on the application and the expected vibration frequency.Alternatively, the length or mass distribution of the piezoelectric beam26, or both, may be altered to tune the piezoelectric beam 26. Thepiezoelectric beam 26 generates a varying or ac voltage when vibrationsare present. Rectifier 16 transforms ac voltage into dc voltage. Anoptional zener diode 30 clamps the output voltage level of the rectifier16 to a desired level. A filter capacitor 32 may be used to smooth orfilter variations in the output voltage of the rectifier 16. The voltageacross the filter capacitor 32 is directly fed to the load 14. Battery18 provides power to the load 14 if the voltage across the filtercapacitor 32 falls below the battery voltage minus the forward voltagedrop of diode 34. This happens when the harvested energy is notsufficient to power the load 14. Diode 34 comes into conduction when thebattery 18 is supplying power to the load 14. When energy harvestingdevice can supply the entire load power, battery 18 does not supplypower to the load 14, during which period, diode 34 prevents accidentalcharging of battery 18. In one embodiment, a Schottky diode 34 is used,which provides a low voltage drop of about 0.3 volts across itself whenin conduction.

When vibrations generated are not high enough to supply all of the loadenergy, diode 34 comes into conduction, and, the piezoelectric source 12will naturally “ring up” (build up at resonance) in voltage to thebattery voltage minus the voltage drop of diode 34. Thus, a portion ofthe load energy will be supplied from the vibrations. Note that evenwith relatively low vibrations the output voltage of piezoelectricsource 12 will ring up to the battery voltage (minus the voltage drop ofdiode 34). This is because the piezoelectric source 12 driven atresonance has a relatively high quality factor (Q). If no energy isdrawn from the piezoelectric source 12, the output voltage can ring upto relatively high values. Thus, the voltage will ring up until someenergy is drawn at the output voltage determined by the battery (minusthe voltage drop of diode 34). When no vibrations are present, thebattery supplies all of the load power.

FIG. 4 is a schematic diagram of an alternative embodiment of a powersystem 10 in accordance with aspects of the present technique. Asdescribed with respect to FIG. 3, the harvested power is rectified,clamped, and filtered by the rectifier 16, zener diode 30, and filtercapacitor 32, respectively. The voltage across the filter capacitor 32is utilized to power a comparator 36. The positive and negative inputsfor the comparator 36 are provided from nodes 38 and 40, respectively.Node 38 is at the ground potential, while node 40 is electricallycoupled to the negative terminal of battery 18. A MOSFET active diodecombination 42 is driven by the output of the comparator 36. In oneembodiment, an n-channel enhancement MOSFET 44 is used in the MOSFETactive diode combination 42, while the parasitic diode 46 of the MOSFETobviates the need for a separate discrete diode. However, a Schottkydiode may be electrically coupled in parallel to the MOSFET active diodecombination 42. The drain of MOSFET 44 is connected to node 40 at thenegative terminal of the battery 18, while the substrate and the sourceterminals of MOSFET 44 are tied with the ground at node 38.

A MOSFET of desirable conduction or on resistance may be chosen forproducing a voltage drop across itself, which facilitates switching ofthe comparator 36. However, if a MOSFET 44 is chosen that does not havesufficient resistance, which is required to switch the states of thecomparator 36, then an optional resistance element may be introducedbetween node 40 and the drain of the MOSFET 44. This resistance elementwill then provide the required voltage drop for switching the comparator36. When the voltage across the filter capacitor 32 is higher than thevoltage across the battery 18 and the MOSFET active diode combination 42in series, the battery may be subject to charging, so that current willflow in the direction from node 40 to node 38 through the MOSFET activediode combination 42. However, this will cause a drop across the MOSFET44, which renders node 40 at a positive potential with respect to node38. However, in the present configuration if node 40 is at a higherpotential than node 38, the output of comparator 36 becomes low. Thislow output of the comparator 36 switches the MOSFET 44 into an offstate, preventing charging of the battery 18.

Conversely, when node 40 is at a lower potential compared to node 38,the output of comparator 36 is positive, which causes MOSFET 44 to comeinto conduction and the battery 18 provides power to the load 14. Thishappens when the voltage across the filter capacitor 32 is notsufficient to keep the battery from supplying power to the load.Therefore, the MOSFET 44 and diode 46 pair prevents accidental chargingof the battery 18 but switches to provide battery power when needed.Furthermore, the MOSFET active diode combination 42 provides a low powerconsumption of less than about five microwatts when in conduction andsupplying tens or hundreds of microwatts.

FIG. 5 is a schematic diagram of another alternative embodiment of thepower system 10, which utilizes an integrated circuit 48. As describedwith respect to FIG. 3 and FIG. 4, the harvested power is rectified,clamped, and filtered by the rectifier 16, zener diode 30, and filtercapacitor 32, respectively. An integrated circuit microcontroller 50 maybe utilized for switching MOSFET 44, which is a p-channel enhancementMOSFET with its parasitic diode 46, in this exemplary embodiment. Oneexample of an integrated circuit microcontroller that may be utilized isLow Loss PowerPath™ Controller (LTC4412) that is commercially availablefrom Linear Technology Corporate of Irvine, Calif. The power inputs forthe microcontroller 50 are provided at the V_(in) and ground (GND) pins.The GATE pin of microcontroller 50 drives the MOSFET 44. When thevoltage level at the SENSE pin is higher than the voltage at the V_(in)pin, the microcontroller 50 will pull up the GATE voltage, thuspreventing MOSFET 44 from coming into conduction. The load is thereforesupplied by the harvested power. In other words, when the harvestedpower is enough to supply the load fully, battery utilization isminimized. However, once the voltage difference between V_(in) and SENSEpins is higher than about 20 mV, the GATE pin of the microcontroller 50is pulled down, thus bringing MOSFET 44 into conduction. This causes thebattery to supply the deficient power to the load.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1. A power system, comprising: an energy harvesting device; a batterycoupled to the energy harvesting device; and a circuit coupled to theenergy harvesting device and the battery, the circuit adapted to controldelivery of power to a load by providing power generated by the energyharvesting device to the load without delivering excess power to thebattery and to supplement the power generated by the energy harvestingdevice with power from the battery if the power generated by the energyharvesting device is insufficient to fully power the load.
 2. The powersystem as recited in claim 1, wherein the energy harvesting devicecomprises a generation device that converts mechanical disturbances intoelectrical power.
 3. The power system as recited in claim 2, wherein thegeneration device comprises a tuning mass configured to mechanicallytune the generation device to a desirable vibration frequency, andconfigured to maximize the power generated by the energy harvestingdevice via resonance.
 4. The power system as recited in claim 1, whereinthe energy harvesting device comprises an acoustic transducer.
 5. Thepower system as recited in claim 1, wherein the energy harvesting devicecomprises a thermal transducer operable to provide electrical power fromthermal gradients.
 6. The power system as recited in claim 1, whereinthe energy harvesting device comprises a transducer operable to provideelectrical energy from light energy.
 7. The power system as recited inclaim 1, wherein the circuit comprises a diode that prevents charging ofthe battery by the energy harvesting device.
 8. The power system asrecited in claim 1, wherein the circuit comprises a comparator coupledto a MOSFET active diode configured to prevent charging of the batteryby the energy harvesting device.
 9. The power system as recited in claim8, wherein the MOSFET active diode is configured to reduce power lossesin the circuit if the battery is supplying power.
 10. The power systemas recited in claim 8, wherein the circuit comprises an integratedcircuit power controller coupled to the MOSFET active diode to preventcharging of the battery by the energy harvesting device.
 11. A powersystem, comprising: a sensor; an energy harvesting device operable todeliver power to the sensor; a battery coupled to the energy harvestingdevice and the sensor; and a circuit coupled to the energy harvestingdevice and the battery, the circuit adapted to control delivery of powerto the sensor by providing power generated by the energy harvestingdevice to the sensor without delivering excess power to the battery andto supplement the power generated by the energy harvesting device withpower from the battery if the power generated by the energy harvestingdevice is insufficient to fully power the sensor.
 12. The power systemas recited in claim 11, wherein the sensor comprises a wireless sensor.13. The power system as recited in claim 11, wherein the sensor iscoupled to a motor and is adapted to provide status data of the motor toa remote monitoring system.
 14. The power system as recited in claim 13,wherein the energy harvesting device comprises a piezoelectric sourcedisposed on the motor that converts vibrations of the motor intoelectrical power.
 15. The power system as recited in claim 13, whereinthe status data comprises data representative of a lifetime of themotor.